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Ice-VII Inclusions in Diamonds: (18)

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Ice-VII Inclusions in Diamonds: (18) RESEARCH GEOCHEMISTRY on the basis of x-ray diffraction data that are presented here (Fig. 1). Ice-VII is a high-pressure form of water ice that is stable above 2.4 GPa Ice-VII inclusions in diamonds: (18). As we show, ice-VII (along with magnesian calcite, ilmenite, and halite) sensitively records high remnant pressures, which then also constrain Evidence for aqueous fluid the pressure and temperature where it has been encapsulated in the host diamond crystal, similar in Earth’s deep mantle to other micrometer-scale inclusions of soft molec- ular materials such as CO2,CO2-H2O, and N2 in diamond (19–23). O. Tschauner,1* S. Huang,1 E. Greenberg,2 V. B. Prakapenka,2 C. Ma,3 G. R. Rossman,3 By retaining high pressures, ice-VII inclusions A. H. Shen,4 D. Zhang,2,5 M. Newville,2 A. Lanzirotti,2 K. Tait6 monitor the former presence of H2O-rich fluid at different depths in the diamond-bearing man- Water-rich regions in Earth’s deeper mantle are suspected to play a key role in the global tle. Remnants of former fluids and melts have water budget and the mobility of heat-generating elements. We show that ice-VII occurs been found as inclusions in many diamonds as inclusions in natural diamond and serves as an indicator for such water-rich regions. through infrared (IR) spectroscopy and micro- Ice-VII, the residue of aqueous fluid present during growth of diamond, crystallizes upon chemical analysis (19, 22). On the basis of IR ascent of the host diamonds but remains at pressures as high as 24 gigapascals; it is spectroscopy, a lower-pressure ice phase, VI, has now recognized as a mineral by the International Mineralogical Association. In particular, been reported as an inclusion in diamond (24), ice-VII in diamonds points toward fluid-rich locations in the upper transition zone and but the fact that aqueous fluid has been trapped around the 660-kilometer boundary. Downloaded from in the TZ or LM and crystallized as ice-VII was previously unknown. he water content of Earth’smantleisa U.Previousmodelshaveproposedthatupwell- Diamonds from southern Africa (Orapa, Nama- key parameter of Earth’s water budget (1). ing mantle releases water while crossing the TZ- qualand), China (Shandong), Zaire, and Sierra Global recycling of water in Earth drives UM boundary and generates a buoyant layer of Leone were examined by diffraction with hard important forms of volcanism such as is- a comparatively water-rich melt that accounts x-rays (0.3344 Å) and a beam focused to 2 mm×3mm T ’ http://science.sciencemag.org/ land arcs (2, 3), controls upper-mantle rhe- for much of Earth s budget of heat-generating at the undulator beamline 13-IDD (GSECARS, ology (4), and plays a role in the evolution of elements (11). Further, downward-moving mate- Advanced Photon Source, Argonne National Lab- mantle plumes (5). Subducted oceanic crust de- rial that crosses the TZ-LM boundary may lose oratory). A PILATUS 3X CdTe1M pixel array de- hydrates at shallow depth (2, 3), whereas faster its chemically bound water upon transformation tector was used for collecting diffraction data. and colder slabs can carry water much deeper into the LM minerals bridgmanite and periclase, Diffraction from ice-VII was observed in dia- (6). Average water abundance in the mantle thus triggering mantle metasomatism around mondsfromsouthernAfricaandChinaasisolated changes markedly across the boundary between the 660-km boundary (10, 11). arrays within the diamond matrix of dimensions the upper mantle (UM) and the transition zone The actual water content of different mantle ranging from 3 mm×10mmtolessthan2mm× (TZ),aswellasacrosstheboundarybetweenthe regions depends not only on thermodynamic lim- 3 mm. Ice-VII was observed as isolated inclusions TZ and the lower mantle (LM). These boundaries its on water solubility but also on the mechanism tens of micrometers afar from other inclusions, originate in the pressure-driven phase trans- in real Earth that carries water to beyond 410 km in proximity to small amounts of nickeliferous formations of mantle rock minerals—in partic- depth (1). The question of water abundance in carbonaceous iron (Fig. 1), similar to metal in- on March 8, 2018 ular, the transformations of olivine to wadsleyite thedeepermantlecanthereforeonlybedecided clusions reported in earlier studies on diamond at 410 km and from ringwoodite to bridgmanite on the basis of actual samples from these re- (24),toilmenite,and,inonecase,toalkalihalides. and periclase at 660 km depth. The UM contains gions whose mineralogical or petrographic record Many inclusions, such as silicates, carbonates, only modest amounts of water on average (7), is placed in context with geochemical and geo- oxides, and halides, were observed through x-ray whereas the average water content in the TZ has physical observations, such as seismic wave at- fluorescence mapping, some of which could be been estimated at ~0.1 weight percent (8), or more tenuation and electrical conductivity (1, 4, 8), clearly identified through diffraction or micro- than 10 times that of the UM. The abundance of and by using experimentally obtained thermo- chemical analysis (Table 1). The observation of water in the LM is unknown, but its constituent dynamic properties. Diamond is the main source olivine with 94 to 97 mol % forsterite compo- minerals appear to have much lower solubility of of minerals from the deeper mantle: A mineral- nent (Fo 94-97) places the origin of the hosting water than the minerals of the TZ (9, 10). The ogical record from depths as great as 660 km or diamonds in mantle peridotite rather than ec- average abundance of chemically bound water even beyond has been identified as inclusions logite, whereas ilmenite is indicative of meta- in these different regions of Earth, as well as pos- in natural diamonds (12, 13). Pearson et al.(14) somatized mantle (15). sible occurrences of smaller layers or loci of water- reported an inclusion of hydrous ringwoodite in In all cases, the diffraction pattern of ice-VII rich rock or melt, are central to our understanding diamondfrombelow520kmdepthwhosefor- is powderlike with no visible granularity (Fig. 1, of Earth’s water budget over extended geologic mation implies a much more hydrous environ- inset). The patterns were unambiguously iden- time. Furthermore, fluids and water-assisted ment than the UM average. Generally, peridotitic tified by Rietveld refinement as those of ice-VII partial melting mobilize mantle-incompatible diamonds mark regions of mantle metasoma- (Table 1 and Fig. 1) (26) and correlate with IR elements, including heat-generating K, Th, and tism (15). Over long geologic time, metasoma- absorption bands of O-H stretching vibrations tism is almost pervasive, at least in Earth’sUM. (26). The high quality of some of the diffraction 1Department of Geoscience, University of Nevada, Las Diamonds conserve important information about data resulted in noticeable proton contributions Vegas, NV 89154, USA. 2Center of Advanced Radiation these important processes (16). to the patterns (Fig. 1). Aqueous fluid in Earth’s Sources, University of Chicago, Chicago, IL 60637, USA. Here, we provide evidence for the presence of mantle is expected to be saline (22)andice-VII 3Division of Geology and Planetary Science, California Institute of Technology, Pasadena, CA 91125, USA. aqueous fluid in regions of the TZ and around can dissolve at least up to ~2 mole percent (mol %) 4Gemological Institute, China University of Geosciences, the TZ-LM boundary by showing the presence of alkali halides (27, 28). For structure analysis Wuhan 430074, China. 5School of Ocean and Earth ofice-VIIasinclusionsindiamondsfromthese and for the assessment of pressure, dissolution ’ Science and Technology, University of Hawai iatManoa, regions of the mantle. Ice-VII has recently been of (Na,K)Cl has been taken into consideration. Honolulu, HI 96822, USA. 6Royal Ontario Museum, Toronto, Ontario M5S 2C6, Canada. recognized as a mineral by the International We provide details about the crystallography of *Corresponding author. Email: [email protected] Mineralogical Association [2017-029 (17)] (Table 1) natural ice-VII in (26); here, we focus on the Tschauner et al., Science 359, 1136–1139 (2018) 9 March 2018 1of4 RESEARCH | REPORT petrologic implications. We note that, with one These inclusions also contain high amounts of phases that have been encapsulated at the same exception (Table 1), the total salinity of the ob- silicate or carbonate (22). Many inclusions of sili- depth are precipitates from one complex fluid. served inclusions (ice-VII plus coexisting phases) cates, carbonates, oxides, and halides are found We propose that a complex aqueous fluid was is much lower than the alkali halide content of within distances of several tens to 100 mmfrom entrapped as separate inclusions that crystal- fluid inclusions commonly observed in diamonds. the ice-VII inclusions. Plausibly, some of these lized as ice-VII, carbonate, halide, and silicate rather than mimicking the bulk fluid composi- tion in each inclusion (26). Fig. 1. Diffraction pat- Because of their confinement by the rigid tern of ice-VII in dia- diamond host crystal, the inclusions of ice-VII mond M57666 from remain at high pressure, allowing us to use the Orapa. Black crosses equation of state of ice-VII to determine mini- are data points; the mum pressures for formation of the surround- Rietveld refinement ing diamond (Table 1). We found pressures of based on this pattern about6GPaand9±1.6GPafordiamondsfrom of the type material Orapa, Botswana. We determined a pressure of (blue curve) converged 12 ± 2 GPa for a diamond from Shandong, to a weighted profile China, and 24 to 25 (±3) GPa for a specimen refinement parameter from Namaqualand. In the fibrous rim of one Rwp of 5.72% with a diamond, we found a NaCl hydrate at a pressure profile refinement of at least 1 GPa, rather than ice-VII (Table 1).
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